This invention relates to magnetic measurement devices and techniques. More particularly, this invention relates to useful devices and methods for measuring the magnetic potential of complex magnetic fields within permanent magnet structures.
A magnetic field at any given point is composed of the x, y and z vector components. Presently, those concerned with measuring a magnetic field within the working cavity of a flux source often use a Hall probe to measure these vector components at discrete points within the field. A composite magnetic field, therefore is obtained by mapping these vector-point measurements together. The accuracy of such magnetic field measurements depends upon the limits of the Hall probe and the precision of placement and orientation of the probe within the magnetic field.
It is not desirable to use Hall probes to perform such measurements because it is extremely difficult and time-consuming to accurately place and orient Hall probes within the magnetic field to obtain the x, y and z vector components for each point""s measurement. Obtaining accurate measurements at many points in a large working cavity suffers from the disadvantages of being tedious, time-consuming and potentially inaccurate. Consequently, those concerned with these magnetic fields within complex, flux source working cavities, have long felt the need for devices and methods to measure such magnetic fields without the burdens, shortcomings and disadvantages associated with Hall probes.
One solution is to measure magnetic potential instead of magnetic field. Magnetic potential is a scalar quantity rather than a three-component vector and since it is scalar it only requires one measurement, rather than three, for each point at which the magnetic field is measured, thereby eliminating the difficulties involved in the precise orientation of Hall probes at many points. Furthermore, measuring magnetic potential instead of the magnetic field allows one to estimate the error pattern. Moreover, both magnetic potential and magnetic field can be easily derived from each other so that if the spatial form of one value is determined, the other value can also be known.
Those skilled in the art know that relative magnetic potential between points can be obtained by measuring the angle of rotation that plane polarized light experiences as it traverses the distance between those points in certain materials placed in a magnetic field. Based on such measurements, the greater the difference in magnetic potential, the more the plane polarized light will rotate per distance within the magnetic field. One way of measuring this rotation is to measure the power loss component of the light traversing a fiber or rod of optically active material through the magnetic field. This can be accomplished by detecting polarized light with a light detecting means having the same planar orientation as the light entering the field. The greater the power loss, the greater the angle of rotation and, as such, the greater the magnetic potential between that point within the field and the point of entry of the light into the optically active material. There are presently no commercially available meters performing that function. The present invention provides such a long-needed measuring device to measure differences in magnetic potential, without suffering from the drawbacks, shortcomings and limitations of Hall probes. To attain this, the present invention uses fiber optic technology to measure the rotation of plane polarized light passing through the magnetic field, whereby the angle of rotation is directly indicative of the field""s potential.
Similarly, there are numerous difficulties involved with field compensation in Magnetic Resonance Imaging (xe2x80x9cMRIxe2x80x9d) magnets. Current MRI procedures place transverse magnetic fields within the interior cavities of magnetic structures surrounding the patient""s body and such transverse magnetic fields must be extremely uniform to provide a clear image, necessitating tedious field compensation techniques. These transverse magnetic fields are created in tubular devices composed of a stack of magnetic frames sufficiently large to surround the patient during the procedure. It is not now possible to manufacture frames with sufficient precision to furnish magnetic fields of requisite uniformity, therefore techniques for correcting field effects of small manufacturing errors is needed. Field correction in general is described in Abele et al., U.S. Pat. No. 5,055,812, entitled xe2x80x9cCompensation For Magnetic Nonuniformities of Permanent Magnet Structures, issued on Oct. 8, 1991. The present invention measures and corrects magnetic potentials rather than magnetic fields. During assembly, the field distribution of the magnetic slices need to be measured, made uniform and then stacked to form the requisite tubular structure.
Based on the same relationship between power loss, angle of rotation and magnetic potential, it is now possible for flawed magnetic fields to be readily and automatically measured by new methods to aid in making field corrections by correct placement of magnetic multipoles. The present invention also provides long-needed methods for automatically field compensating using a stationary probe array for numerous magnetic potential measurements from a series of magnetic slices and dipoles, without suffering from the drawbacks, shortcomings and limitations of Hall probes. The present invention also provides an improved dipole compensating method for comers of magnet slices which performs those long-needed functions without suffering from the drawbacks, shortcomings and limitations of Hall probes.
It is an object of the present invention to provide highly accurate devices utilizing fiberoptics for detecting the rotation of plane polarized light and measuring magnetic potential of complex magnetic fields located within the working cavity of a magnetic flux source.
It is another object of this invention to provide a field compensating method employing a stationary probe array.
It is a further object of this invention to provide a dipole compensating method to automatically compensate for flawed magnetic field sources by taking numerous magnetic potential measurements from a series of magnetic slices and dipoles within a stationary array of probes.
To attain the field compensation method using a stationary probe array, the methods of the present invention contemplate forming an array of stationary magnetic probes positioned in the same configuration as the points to be tested in a cross section of the magnetic frame. The magnetic frame is then placed over the array of magnetic probes. This invention""s method further comprises making numerous measurements automatically and provided the data from these measurements to a data processing means in order to map the magnetic potential of the magnetic frame so that the magnetic strength and orientation of the compensating dipoles can be adjusted within the test array.
To attain the field compensation method using a stationary probe array, the methods of the present invention contemplate forming an array of stationary magnetic probes positioned in the same configuration as the points to be tested in a cross section of the magnetic frame. The magnetic frame is then placed over the array of magnetic probes. This invention""s method furhte comprises making numerous measurements automatically and provided the data from these measurements to a data processing means in order to map the magnetic potential of the magnetic frame so that the magnetic strength and orientation of the compensating dipoles can be adjusted within the test array.
The mapping devices of this invention can be used in magnetic field mapping in complex, high uniformity field devices such as MRI""s, electron beam devices and numerous other applications. The field and dipole compensating methods can be used for magnetic field mapping in complex, high uniformity field devices such as MRI""s and numerous other applications.
The exact nature of this invention, as well as other objects and advantages thereof, will be readily apparent from consideration of the following specification relating to the annexed drawings.